The most common process used for the production of pulp and paper is known as the Kraft process. J. Gierer, Wood Sci. Technol. 14, 241-266 (1980). In this operation, wood chips and various chemicals known as white liquor are cooked in a digester to produce pulp and a residual black liquor.
The black liquor contains wood components, including lignin, dissolved during digestion and also contains inorganic materials such as sodium sulfate and sodium carbonate. The black liquor leaving the digester has a concentration of about 15 percent total solids. In order to burn efficiently in a recovery boiler, the liquor must be concentrated, usually in multiple-effect evaporators, to about 65 percent total solids. The liquor is then burned in the recovery boiler where further concentration of the liquor and combustion of the liquor solids occur. The inorganic components are recovered from the liquor in the boiler, redissolved in water, and then causticized to regenerate the white liquor used to digest the wood chips in the first step of the process.
In order for this process to be cost effective, however, the cooking chemicals which react and become a part of the black liquor must be efficiently recovered. J. Smook, Handbook for Pulp and Paper Technologists, 2nd ed., 1992, Chapt. 7 and 10. As mentioned previously, in order to regenerate these chemicals, the black liquor is subjected to several treatments that include evaporation and burning. These two steps play a crucial role due to their large energy consumption. Because the costs of energy have greatly increased in recent years, the pulp and paper industry has begun processing black liquor with higher solids concentrations in the recovery boiler. L. Soderhjelm, Paperi ja Puu 9, 642-652 (1986). Higher solids enhance recovery boiler performance by providing increased boiler capacity, lower emissions, and higher thermal efficiency. M. Boone, 1991 Tappi Kraft Recovery Operations Short Course, 93-97 (1991). The ability to concentrate black liquor is limited, however, by its rheological (deformation and flow) properties. Black liquor shows an exponential increase in viscosity as its solids content rises P. Ramamurthy, A. Mujumdar, A. van Heiningen, G. Kubes, Tappi, 195-202 (April 1992)), thereby preventing its processability in terms of transport, storage, and handling. As a result, black liquor having a solids content greater than about 70 percent has too high a viscosity to pump or otherwise process.
Black liquor is a complex aqueous system made up of several components including, polysaccharides, lignin, salts, and fatty acids. Previous research has reported conflicting results as to which of these components may be responsible for the elevated viscosity. Both the lignin (J. Small, A. Fricke, Ind. Eng. Chem. Prod. Res. Dev. 24, 608-614 (1985)) and the polysaccharide (L. Soderhjelm, 1989 Int'l. Chem. Recovery Conf. Proc., 95-99 (1989)) fractions may be the cause. Nonetheless, strong (or concentrated) black liquor becomes a sticky, unpumpable liquid with increasing solids content. M. Boone, 1991; P. Ramamurthy et al., April 1992. Even with less concentrated, more pumpable liquor, pluggage and scaling in the recovery boiler are a critical issue. R. Ryham, S. Nikkanen, 1992 Kraft Recovery Operations Short Course, 222-238 (1992).
In order to prevent the problems described above, methods such as heat treatment (L. Soderhjelm, 1986; R. Ryham, 1992 Int'l. Chem. Recov. Conf., 581-588 (1992); L. Soderhjelm, Appita 41, No. 5, 299-392 (1988)) and oxidation (E. Milanova, G. Dorris, Jour. of Pulp and Paper Science 16, No. 3, J94-J101, (1990); W. Frederick, T. Adams, 1992 Kraft Recovery Operations Short Course, 97-112 (1992)) are being used to reduce the viscosity of black liquor and obtain a higher solids content. In the heat treatment process, the liquor is removed from the evaporators and heated at an elevated temperature for an extended period of time (i.e. about 140.degree. C. for 2 hours) (L. Soderhjelm, 1988). This procedure results in an irreversible decrease in viscosity due to a depolymerization of the large polymer chains present within black liquor. R. Ryham et al., 1992. In the oxidation process, the liquor is exposed to air in order to convert the sulfide in black liquor to thiosulfate. E. Milanova et al., 1990. This oxidation reaction provides a reduction in viscosity by lowering the residual alkali concentration. This reduction is, however, reversible, and addition of alkali following the oxidation will return the liquor to its original viscosity.
Both heat treatment and oxidation have significant disadvantages. They may cause an increase in viscosity if the black liquor has an initially low residual alkali. W. Frederick et al., 1992. Both processes are also cost intensive and may not be worth the advantage that they provide. For example, heat treatment requires energy to raise the temperature of the liquor. Similarly, oxidation rids the liquor of its fuel value which requires more energy input. J. Smook, 1992. A more advantageous procedure for reducing the viscosity of black liquor to obtain a processable, high solids liquid is needed.
In addition to heat treatment and oxidation, other methods exist for lowering the viscosity of alkaline waste liquor, including black liquor. U.S. Pat. No. 4,911,787 discloses a method and an apparatus for concentrating black liquor wherein CO.sub.2 gas is added to the black liquor to reduce its boiling point and its viscosity. The disadvantages of this method include the requirement of additional equipment needed to dissolve the CO.sub.2 in the liquor. Because the addition of CO.sub.2 to black liquor results in the generation of H.sub.2 S, an additional oxidation step such as adsorption, membrane separation, or low temperature processing may be required. Furthermore, the absorption of CO.sub.2 in quantities necessary for viscosity reduction is adversely limited by the temperature and pH of the liquor.
U.S. Pat. No. 4,734,103 discloses a high solids black liquor having the property of reduced turbulent flow/drag comprising a high solids black liquor which contains a few parts per million of a water soluble terpolymer such as acrylic acid-acyrylamide-sulfo lower alkyl acrylamide polymer. Additional equipment is needed to produce the polymers which are added to the black liquor.
The present invention results in a high solids content black liquor of much lower viscosity than enabled by the methods disclosed above.